I thought it might help out future users to see a compilation of acceptance rates. This statistic might have been revealed to some of you who attended graduate student visit days.
I only attended 3 visit weekends, so please add to this if you can. Include university name, department, year and semester you applied for, and acceptance rate.
University of Michigan
Aerospace Engineering - <10% for Fall 2014 (13% for Fall 2013)
Cornell University
Mechanical and Aerospace Engineering - ~12% Fall 2014
Northwestern
Mechanical - I recall in the high teens, but I'm not very positive.